1. Field of the Invention
The present invention relates to a substrate for a semiconductor element. The semiconductor element is mounted on the substrate. In particular, the present invention relates to a method for manufacturing a substrate which has structural characteristics that are similar to those of a lead frame. The present invention also relates to a semiconductor device using the substrate.
2. Description of the Related Art
Various types of semiconductor elements such as memory, CMOS, CPU, and the like, are manufactured by a wafer process. These semiconductor elements have a terminal for electrical connection. The magnitude of the pitch of the terminal for electrical connection is different from the magnitude of the pitch of the connection part at a print substrate side by approximately several to several hundred times. A semiconductor element is attached to the connection part at the print substrate side. Therefore, when the semiconductor element is about to be connected with the print substrate, an intermediary substrate (a substrate for mounting a semiconductor element) called an “interposer” is used for pitch conversion.
The semiconductor element is mounted on one side of this interposer. A connection with the print substrate is made at another surface or a peripheral of the substrate. The interposer includes a metallic lead frame in the interior or at a front surface. An electrical connection channel is routed by the lead frame. In this way, the pitch of an external connection terminal is expanded. The external connection terminal makes a connection with the print substrate.
FIGS. 2A-2C are schematic diagrams showing a structure of an interposer using a QFN (Quad Flat Non-lead) type lead frame, which is an example of a conventional interposer.
As shown in FIG. 2A, a flat part 21 of a lead frame is provided at a central part of a lead frame. The lead frame is formed primarily of either aluminum or copper. A semiconductor element 22 is mounted on the flat part 21 of the lead frame. A lead 23 with a wide pitch is placed at an outer peripheral part of the lead frame. A wire bonding method is used to connect the lead 23 and the terminal for electrical connection of the semiconductor element 22. The wire bonding method uses a metal wire 24 such as an Au line and the like. As shown in FIG. 2B, an overall integration is made at a final stage by performing a molding process with a molding resin 25.
Incidentally, a holding material 27 shown in FIGS. 2A and 2B is used to hold a lead frame. The holding material 27 is removed as shown in FIG. 2C, after a molding is performed with the molding resin 25. However, according to the interposer shown in FIGS. 2A-2C, an electrical connection can be made only at an outer peripheral part of the semiconductor element and an outer peripheral part of the lead frame. Therefore, there is a problem in that the interposer is not suitable for a semiconductor element having a large number of terminals.
When the semiconductor element has a small number of terminals, the connection between the print substrate and the interposer is conducted by attaching a metallic pin on an extraction electrode 26 at an outer peripheral part of the interposer. Furthermore, when the semiconductor element has a large number of terminals, a BGA (Ball Grid Array) is used. According to the BGA, a solder ball is positioned in an array pattern at an external connection terminal at an outer peripheral part of the interposer.
According to a semiconductor element having a small area and a large number of terminals, it is difficult to convert a pitch when an interposer has only one layer of wiring layer. Therefore, a procedure is often conducted to increase the number of wiring layer included in the interposer, thereby stacking a plurality of wiring layers.
A connection terminal of a semiconductor element, having a small area and a large number of terminals, is often formed at a bottom surface of the semiconductor element by being placed in an array form. Therefore, a flip chip connection method is often used. According to this flip chip connection method, an external connection terminal at an interposer side is placed in an array form which is the same as a connection terminal of a semiconductor element. Furthermore, according to this flip chip connection method, a minimal amount of solder ball is used to connect the interposer and the print substrate. The wiring inside the interposer is created by forming a hole from an upper part in a perpendicular direction with a drill or a laser and the like, forming a metallic plating inside the hole, and thereby creating electrical conductivity between the upper and lower layers.
According to an interposer based on this method, the pitch of the external connection terminal may be made small to approximately 150 to 200 μm. As a result, it is possible to increase the number of connection terminals. However, the reliability and stability of the connection are reduced. Thus, the above configuration is not suitable for mounting on a vehicle, which requires a high degree of reliability.
Several types of interposers have been designed. The material used to create the interposer and the structure of the interposer are different. For example, an interposer is configured so that ceramic is used in the structure of a portion holding the lead frame part. Another type of interposer is configured so that the base material of the interposer is an organic substance such as P-BGA (Plastic Ball Grid Array), CSP (Chip Size Package), or LGA (Land Grid Array). These interposers are utilized as appropriate according to actual use and required configurations.
As the size of semiconductor elements become smaller, as the number of pins increases, and/or as the speed of the semiconductor elements increases, adjustments are made by the interposers described above. For example, a fine pitching and an adjustment to high speed signals are made. The fine pitching is made to the connection part connecting with the semiconductor element of the interposer. Taking into consideration that the pitch has become more and more minute, it is necessary that the pitch of a terminal portion of recent interposers be approximately 80 to 100 μm.
Incidentally, the lead frame is used as a conduction part and a supporting component. As a representative example, the lead frame is formed by applying an etching process on a thin metal plate. It is preferable that the thickness of the metal plate be equal to approximately 120 μm, so that the etching process may be performed with stability, and so that an appropriate handling is made in the procedures after the etching process. Furthermore, a certain level of thickness and a land area is required for the metallic layer to contribute to an adequate amount of joint strength during the wire bonding process.
Taking these conditions into consideration, it is necessary that the thickness of the metal plate for the lead frame be at least approximately 100 to 120 μm. Furthermore, in this case, when an etching processing is performed from both sides of the metal plate, it is believed that the pitch of the lead may be minimized to approximately 120 μm, while the width of the lead line may be made finer to approximately 60 μm.
Another problem is that, during a process of manufacturing an interposer, it is necessary to discard the holding material. This discarding procedure is believed to lead to an increase in costs. An explanation in this regard is provided below using FIGS. 2A-2C.
The lead frame is attached to the holding material 27 made of polyimid tape. A semiconductor element 22 is fixed to a flat part 21 of the lead frame with a fixing resin or a fixing tape 28. Thereafter, a wire bonding is performed. According to the transfer mold method, a plurality of chips, i.e., the semiconductor element 22, are integrally molded by the molding resin 25. Thereafter, an external processing is performed. A cutting is made so that each interposer becomes independent.
When a back surface 29 of the lead frame becomes a connection surface connecting with a print substrate, it is necessary to prevent the molding resin 25 from wrapping around a connection terminal surface of a back surface 29 of the lead frame and sticking to the connection terminal during molding. Therefore, the holding material 27 has been necessary in a process manufacturing an interposer. However, in the end, the holding material 27 is unnecessary. Thus, after a molding procedure is performed, it is necessary to remove the holding material 27 and discard it. This leads to an increase in costs.
Japanese Unexamined Patent Application, First Publication No. H8-340069 shows an example of such a conventional technology. According to Japanese Unexamined Patent Application, First Publication No. H8-340069, an insulating resin supports a conducting post which penetrates a substrate. Thus, Japanese Unexamined Patent Application, First Publication No. H8-340069 shows a configuration in which the conducting post part is sticking out from a resin.
Japanese Unexamined Patent Application, First Publication No. H8-340069 shows an example of a method providing a substrate for a semiconductor element which solves the problems described above, allows the formation of a wiring with an extremely small pitch (i.e., a wiring with an ultrafine pitch), enables a wire bonding processing in a stable manner, and is cost-effective. According to the method described in Japanese Unexamined Patent Application, First Publication No. H10-223828, the substrate for a semiconductor element is structured so that a premold resin layer is a supporting body of a wiring.
Hereinafter, a method, disclosed in Japanese Unexamined Patent Application, First Publication No. H10-223828, of manufacturing a substrate for a semiconductor element in the form of a lead frame is described. A resist pattern for forming a connection post is created on a first surface of a metal plate. A resist pattern for forming a wiring pattern is created on a second surface of the metal plate. An etching procedure is conducted on the copper from above the first surface to a desired thickness. Thereafter, a premold resin is applied to the first surface, thereby forming a premold layer. Then, an etching procedure is conducted from the second surface, a wiring is formed, and finally, the resist on both sides are peeled off.
According to the substrate for a semiconductor element in the form of a lead frame, manufactured as described above, when the thickness of the metal is made as thin as possible to a level at which a fine etching process is possible, an etching procedure can be performed in a stable manner because the premold resin is acting as a supporting body. Furthermore, since the scattering of an ultrasonic wave energy is small, the wire bonding characteristics are superior as well. In addition, since a holding material such as a polyimid tape is not used, it is possible to reduce the cost used for the holding material.
However, there is a problem in the technology described in Japanese Unexamined Patent Application, First Publication No. H10-223828. According to the technology described in Japanese Unexamined Patent Application, First Publication No. H10-223828, the thickness of the premold resin that is filled in must be thick enough to provide the necessary rigidity to the lead frame. At the same time, the bottom surface of the connection post must be completely exposed.
A concrete solution for applying a resin while controlling the thickness is, for example, a method in which a syringe and the like is used to pour resin into one point of a bottom of an applied surface, and wait until the resin permeates the entire applied surface. In this case, the filling-in process of the premold resin must be stopped before the height of the premold resin reaches the bottom surface of the connection post. This is because it is necessary to maintain a condition in which the bottom surface of the connection post is exposed. As a result, the height of the premold resin must become lower than the connection post.
When a substrate for a semiconductor element is completed while maintaining this condition, the connection post is sticking out from the surrounding premold resin layer. When a solder ball is mounted on this, for example, it is possible that the ball drops down from the connection post due to a minor deviation in the position at which the ball is mounted. Consequently, the yield is reduced.
The present invention is made according to the problems described above. Thus, the present invention provides a semiconductor device and a method for manufacturing a substrate for a semiconductor element, which prevents a solder ball from dropping from a land when the solder ball is mounted, and which allows the solder ball to be mounted at a high yield during a process of manufacturing a substrate for a semiconductor element shaped like a lead frame provided with a premold.